1. Field of the Invention
One disclosed aspect of the embodiments relates to a driving apparatus for a vibration-type actuator. In particular, one embodiment relates to a driving apparatus for a vibration-type actuator in which a unit that generates an AC voltage applied to an electro-mechanical energy conversion element is a pulse generation unit and an amplitude of the AC voltage is changed by adjusting a pulse width.
2. Description of the Related Art
As one form of a vibration-type actuator, a vibration-type actuator including a plurality of piezoelectric elements whose polarities are alternately inverted, a vibration member adhering to the piezoelectric elements, and a movable member pressed against and brought into contact with the vibration member is known. In this vibration-type actuator, a two-phase AC signal, in which each phase has a phase difference of 90° from each other, is applied to each piezoelectric element. Accordingly, a traveling wave is generated on the vibration member, and the movable member may obtain a driving force by a frictional force. The vibration-type actuator is advantageous in that a driving torque at a low speed is great and a response is good, as compared to an actuator using an electromagnetic force.
In general, as a method for controlling a rotation speed or a position of the vibration-type actuator, a method for controlling any one or a combination of a frequency, a voltage amplitude, and a phase difference of an AC voltage is known. In particular, the frequency-based control in which a dynamic range is wide and speed control is easy is the most common control scheme. However, the voltage amplitude-based control and the phase difference-based control considering their characteristics have also been discussed as control schemes. The voltage amplitude-based speed control exhibits a stable control characteristic, particularly in low-speed control, and is effective in high-precision position control, as compared to the frequency-based control. However, a method for changing amplitude of a pulse signal by controlling a supply voltage or a method for changing a gain of an amplifier is necessary to change the voltage amplitude, and there is a problem that a circuit becomes complicated.
As solutions to such an issue, several schemes of changing a pulse width of a pulse signal to change an amplitude of an AC voltage applied to a vibration-type actuator has been discussed. Japanese Patent Application Laid-Open No. 63-305771 discusses the following vibration-type actuator.
Specifically, Japanese Patent Application Laid-Open No. 63-305771 discusses a configuration in which a pulse signal is produced using a known triangular wave comparison scheme PWM modulation, and a voltage corresponding to a desired voltage amplitude is compared with a triangular wave at a higher frequency than a driving frequency to thereby output a signal having a desired voltage amplitude. Further, Japanese Patent Application Laid-Open No. 2007-189823 discusses a technique in which when speed of a movable member is controlled, voltage command value is determined based on a state amount of a vibration-type actuator and a command value of the state amount and a pulse width of a pulse signal is linearly changed according to a change in the voltage command value.
However, in Japanese Patent Application Laid-Open No. 63-305771, the pulse signal is generated at a higher frequency than a driving frequency to linearly change an amplitude of an applied voltage according to a change in command value of a voltage amplitude. Accordingly, there are problems in which a loss increases and a driving circuit becomes complicated due to switching when the pulse signal is generated. Further, in Japanese Patent Application Laid-Open No. 2007-189823, there is a problem in which since the amplitude of the driving voltage is not linearly changed according to a change in a pulse width, an AC voltage having an amplitude according to a command value may not be applied to the vibration-type actuator.